A crankshaft is a principal component of a reciprocating engine, which produces power by converting reciprocating motion of pistons to rotary motion. Generally, there are two types of crankshafts: one that is manufactured by die forging and one that is manufactured by casting. For engines of automobiles such as passenger cars, freight cars, and specialized work vehicles, particularly multiple cylinder engines having two or more cylinders, it is necessary that their crankshafts have high strength and stiffness, and therefore die forged crankshafts are widely used. For engines of motorcycles, agricultural machines, and marine vessels, die forged crankshafts are also used. Further, for reciprocating compressors and reciprocating pumps, in which power transmission occurs in a reverse direction to that of a reciprocating engine, i.e., rotary motion is converted to reciprocating motion, die forged crankshafts are also used.
In general, as described in Patent Literatures 1 and 2, for example, die forged crankshafts for use in such reciprocating apparatus are manufactured in the following manner a billet having a circular or square cross section and having a constant cross-sectional area along the entire length is prepared as a starting material, and the billet is subjected to the steps of preforming, die forging, trimming and coining in order. The preforming step includes roll forming and bend forging, and the die forging step includes block forging and finish forging.
FIG. 1 is a diagram schematically showing a typical conventional process for manufacturing a die forged crankshaft. A crankshaft 1 illustrated in FIG. 1 is intended to be mounted in a 4-cylinder engine. It is a 4-cylinder 8-counterweight crankshaft that includes: five journals J1 to J5; four crank pins P1 to P4; a front part Fr, a flange Fl, and eight crank arms (hereinafter referred to as “crank arms”) A1 to A8 that connect the journals J1 to J5 and the crank pins P1 to P4 to each other, wherein each of the eight crank arms A1 to A8 has a balance weight. Hereinafter, when the journals J1 to J5, the crank pins P1 to P4, and the crank anus A1 to A8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the crank pins, and a reference character “A” for the crank arms.
In the manufacturing method shown in FIG. 1, a billet 2 shown in FIG. 1(a), which has been previously cut to a predetermined length, is heated by an induction heater or a gas atmosphere furnace and then is subjected to roll forming. In the roll forming step, the billet 2 is rolled and reduced in cross-section by grooved rolls, for example, to distribute its volume in the longitudinal direction, whereby a rolled blank 103, which is an intermediate material, is formed (see FIG. 1(b)). In the bend forging step, the rolled blank 103 obtained by roll forming is partially pressed in a press in a direction perpendicular to the longitudinal direction to distribute its volume, whereby a bent blank 104, which is a secondary intermediate material, is formed (see FIG. 1(c)).
In the block forging step, the bent blank 104 obtained by bend forging is press forged with a pair of upper and lower dies, whereby a forged blank 6 having a general shape of a crankshaft (forged product) is formed (see FIG. 1(d)). Furthermore, in the finish forging step, the block forged blank 6 obtained by block forging is press forged with a pair of upper and lower dies, whereby a forged blank 7 having a shape that corresponds to the shape of the crankshaft is formed (see FIG. 1(e)). In the block forging and the finish forging, excess material flows out as a flash from between the parting surfaces of the dies that oppose each other. Thus, the block forged blank 6 and the finish forged blank 7 include flashes 6a, 7a, respectively, around the formed shape of the crankshaft.
In the trimming step, the finish forged blank 7 with the flash 7a, obtained by finish forging, is held by dies from above and below and the flash 7a is trimmed by a cutting die. In this manner, a die forged crankshaft 1 is obtained as shown in FIG. 1(f). In the coining step, principal parts of the die forged crankshaft 1, from which the flash has been removed, e.g., shaft parts such as the journals J, the crank pins P, the front part Fr, and the flange Fl, and in some cases the crank arms A, are slightly pressed with dies from above and below and formed into a desired size and shape. In this manner, the die forged crankshaft is manufactured.
The manufacturing process shown in FIG. 1 is applicable not only to a 4-cylinder 8-counterweight crankshaft as exemplified, but also to a 4-cylinder 4-counterweight crankshaft in which, among 8 crank arms A, the leading crank arm A1, the trailing crank arm A8, and the two central crank arms A4, A5 have balance weights. Also, the same manufacturing process can be applied to crankshafts that are to be mounted in a 3-cylinder engine, an inline 6-cylinder engine, a V-type 6-cylinder engine, an 8-cylinder engine, and the like. It is noted that, when adjustment of the placement angle of the crank pins is necessary, a step of twisting is added after the trimming step.